1. Field of the Invention
The present invention relates to a method and a device for detecting the motor position of a synchronous motor or of a brushless DC motor.
To drive a DC motor, it is known for the motor windings to be optionally connected to a first or second drive potential through respective half-bridge circuits each having two series-connected semiconductor switches. The individual semiconductor switches are driven in a predetermined, periodically recurring “drive pattern” in order to generate a rotating field that brings about a motor rotation. “Drive pattern” designates the totality of the mutually coordinated temporal profiles of the switching states of the semiconductor switches or of the drive signals for the semiconductor switches used for driving the motor. In the case of a three-phase motor, three half-bridges for driving the motor include a total of six semiconductor switches; the drive pattern, thus, encompasses the time profiles of the drive signals of these six semiconductor switches. The period with which the drive patterns recur in this case corresponds to the time duration in which the rotating field completes one revolution. The mechanical rotation that the rotor of the motor experiences during a revolution of the rotating field is dependent on the number of pole pairs of the motor. The relationship whereby the angle of a mechanical rotation of the rotor corresponds to the quotient of the angle of the rotating field (the electrical revolution) and the number of pole pairs holds true in this case.
So that the drive pattern can be generated in a suitable manner, it is necessary to detect the angular position of the motor.
For driving the motor coils there are various methods that effect various drive patterns. In some of these methods, such as the so-called block commutation, for example, the switches are driven such that there is always one of the motor windings that is not connected to one of the supply potentials during a time period within the drive cycle. To detect the motor position, it is known, in the case of this type of driving, to detect the back-induced voltage in the motor winding that is currently connected to no voltage supply. This back-induced voltage has a periodic time profile with a period duration that depends on the rotational speed of the motor or on the time duration per revolution. The back-induced voltage intersects a voltage value representing a zero point of the motor twice per period, the position of these zero crossings containing a precise item of information about the angle of position of the motor. In the case of a three-phase motor, the temporal profiles of the back-induced voltages are shifted relative to one another in each case by a value that corresponds to a revolution of the rotating field through 120°. Because each of these back-induced voltages has two zero crossings per revolution of the rotating field or one zero crossing per half revolution, the motor position can be determined from these three back-induced voltages with a resolution of 60° relative to one revolution of the rotating field. In such a case, the resolution capability with regard to determining the mechanical position of the motor rises with the number of pole pairs. While the motor position can be determined with a resolution of 60° in the case of one pole pair, a resolution of 30° is already possible in the case of two pole pairs, etc.
Because the zero point of a motor is, generally, not accessible for tapping off the back-induced voltages across the motor windings, it is known to simulate the zero point of the motor, for example, by a star circuit with resistors, and to use as back-induced voltage the potential difference between the accessible terminal of the respective motor winding and the simulated zero point. This method is described for example in Oswald, Wagner, Wasson: “The Brushless Spindle Motor: A background in the motors, magnetics, electrical circuits, and control systems”, pages 23 to 26. Jufer, Osseni: “Back EMF Indirect Detection for Self-Commutation of Synchronous Motors”, European Power Electronics, Grenoble, September 22 to 24, pages 1125 to 1129, discloses calculating the third harmonic of the back-induced voltage to deduce the motor position therefrom.
FIG. 1 shows a circuit diagram of a motor with a circuit configuration for determining the motor position according to the prior art.
The motor is represented by three coils L1, L2, L3 that are connected to a common zero point N by one of their terminals in each case in a star circuit. Three half-bridge circuits 10, 20, 30 are present for driving the motor, which half-bridge circuits each include a series circuit formed by a first semiconductor switch (high-side switch) H1, H2, H3 and a second semiconductor switch (low-side switch) L1, L2, L3, which are connected between a terminal for a first supply potential VS and a terminal for a second supply potential or reference-ground potential GND. Each of the half-bridges 10, 20, 30 has an output A, B, C, an output A, B, or C in each case being connected to one of the motor windings or one of the three terminals of the motor.
The profile of drive signals in_H1, . . . , in_L3 for the switches H1, . . . , L3 in the case of a block commutation is illustrated in FIG. 2A for a time duration corresponding to a revolution of the motor through 360°+60°. The illustration of the drive signals is chosen such that the respective semiconductor switch H1, . . . , L3 is in the on state if the associated drive signal in_H1, . . . , in_L3 assumes a high level, and that the respective semiconductor switch is in the off state if the associated drive signal assumes a low level. In FIG. 2A, in_H1 designates the drive signal of the high-side switch H1 of the first half-bridge 10, in_L1 designates the drive signal of the low-side switch L1 of the first half-bridge, in_H2 designates the drive signal of the high-side switch H2, in_L2 designates the drive signal of the low-side switch L2, in_H3 designates the drive signal of the high-side switch H3, and in_L3 designates the drive signal of the low-side switch L3.
During the block commutation, at an arbitrary point in time, in each case a maximum of one of the high-side switches H1, H2, or H3 and only a maximum of one of the low-side switches L1, L2, or L3 are in the on state, the semiconductor switches of the same half-bridge usually never being in the on state simultaneously. Through a suitable temporal change of the switching states of the semiconductor switches H1, L1, H2, L2, H3, L3 and a resultant change of the phase currents in the motor windings, a rotating field arises that effects a rotation of the rotor in the motor. In FIG. 2A, the temporal sequence of the drive signals per motor revolution is subdivided into six time segments that each correspond to a motor rotation through about 60° and during which in each case one of the high-side switches and one of the low-side switches undertake the driving of the motor. These are the high-side switch H1 and the low-side switch L2 during a first time period designated by “1”, the high-side switch H3 and the low-side switch L2 during a time period designated by “2”, the high-side switch H3 and the low-side switch L1 during a period designated by “3”, etc. This drive pattern recurs starting from the segment designated by “7”, in which the driving of the switches H1, . . . , L3 corresponds to that in the segment “1”.
During one of these time periods, during which always the same high-side switch and the same low-side switch undertake the driving of the motor, a current flows through two of the windings, namely, the coils that are connected to the half-bridges whose semiconductor switches are in the on state. These are the windings 1 and 2, for example, in the time segment “1”. A back-induced voltage EA, EB, EC can, then, be tapped off across the respective non-energized winding (winding 3 during the time segment “1”), the time profile of which voltage is illustrated in FIG. 2B. The time profiles of the back-induced voltages across the windings 1, 2, 3 are periodic with a period duration that corresponds to a complete revolution of the motor, and are offset relative to one another in each case by a value that corresponds to a revolution of the rotating field through about 120°. The time profiles of the back-induced voltages intersect a zero line, which corresponds to the potential at the motor zero point N, in each case twice per period. With knowledge of the zero points of the back-induced voltages EA, EB, EC of all three coils, the motor position can be determined with an accuracy of 60°.
Because the motor zero point N usually cannot be contact-connected, it is known to simulate the zero point, for example, by a resistor network RN, and to compare the potentials at the terminals of the motor with the potential of the simulated zero point NN, as is illustrated in FIG. 1.
To set the current flowing into the motor, usually only one of the two switches that undertake the driving of the motor during one of the segments “1”, “2”, etc., is driven into the on state permanently, while the other is driven in pulsed or pulse-width-modulated fashion. In the example in accordance with FIG. 2A, the high-side switch H1, H2, H3 is in each case driven into the on state permanently, while the low-side switch is driven in clocked fashion. The current consumption of the motor windings energized by the two switches is, then, dependent on the duty ratio with which the low-side switch is respectively driven. What is problematic in such a case is that a high-frequency interference voltage is superposed on the back-induced voltage as a result of the clocked driving of one of the two switches, which interference voltage makes it considerably more difficult to evaluate the voltage that is to be measured. To enable the back-induced voltage to be evaluated, it is known to filter the potential at the terminal or the back-induced voltage of the non-energized winding before the comparison with the potential of the simulated zero point using a low-pass filter, as is illustrated for one of the windings in FIG. 1. While a simple RC filter is represented in FIG. 1 for illustration purposes, in practice, very complex filters are necessary to achieve a satisfactory result in the filtering of the back-induced voltage and, thus, in the determination of the motor position.
Such filters have to be constructed externally and lead not only to considerable costs but also to a significant signal deformation, for example, to a speed-dependent phase shift.